1. Field of the Invention
This invention relates generally to a method and apparatus for foaming one or more liquid products. In particular, this invention relates to apparatus and methods involving aspirators with a compound exit angle on the discharge side of the foamer, thereby maintaining maximum efficiency across the aspirator throat, while permitting the use of a wide range of inlet pneumatic and hydraulic pressures.
2. Description of the Related Technology
Foams consist of a mass of gas bubbles dispersed in a liquid. The bubbles are separated from each other by thin films of liquid, most of the volume being attributable to the gas phase.
The desirable characteristics of foams depend upon their application. For example, shampoos and bubble bath compositions form slow draining and persistent foams. For fire fighting, the foam should resist destruction by contact with fuel and exposure to high temperatures. On the other hand, for laundering and washing machines, too much foam should be avoided. The mechanics of foam formation has therefore evolved into a subject of considerable technical importance.
Properties of foams are influenced by a variety of factors, such as the extent of adsorption from solution at liquid gas interfaces, the rheological characteristics of the adsorbed films, gaseous diffusion, bubble size distribution and temperature. Foam properties are primarily dependent upon the chemical composition and characteristics of the adsorbed films. Foaming properties cannot be related or described by a single specific property or attributed to one constituent of a multi-component composition.
The various methods for making foam differ mostly in the way the gas is introduced into the solution. The most common methods consist of bubbling gas through orifices, by the use of injectors, by agitation, or by various other mechanical means, as well as by chemical generation of gas in the liquid. The basic apparatus used to form foam typically consists of a mixing chamber into which the foaming material is introduced by means of one or more nozzles. Some mechanism is then provided to facilitate the entrainment of air, thereby converting the solution into foam, which then usually goes through disks or some sort of mechanical atomizer that disperses the foam into smaller bubble sizes. In such an apparatus, a carrier liquid under pressure typically passes through a restricted throat which opens into an expansion chamber, the chamber being coaxial with the throat. The conduit for introducing the second liquid into the device usually enters from the side, such that suction created in the expansion chamber forces the second liquid to enter the main carrier stream where mixing takes place. A foam producing apparatus of this type is disclosed in U.S. Pat. No. 2,571,871, which discloses a cylindrical throat which opens abruptly into a coaxial cylindrical expansion chamber of a larger cross section, with a conduit entering the expansion chamber from the side adjacent the junction of the throat and chamber. The proper proportions of the constituents of the foam are maintained by the introduction of a screen or perforated disk placed at the discharge end, thereby offering some resistance to the discharging stream.
Ideally, the turbulent mixing action of air and liquid within the expansion chambers should be sufficient in itself to produce a foam of the desired consistency, but the use of wire screens, perforated plates or fibrous materials has in the past been necessary to improve the breaking up of the initial mixture into a substantially uniform foam. A device which has multiple chambers of fibrous materials coupled with perforated plates is disclosed in U.S. Pat. No. 2,715,045. It should be noted that the foamer disclosed in U.S. Pat. No. 2,715,045 uses high pressure air to entrain a liquid foaming product. Most modern foamers, by contrast, utilize a liquid product stream and either entrain or otherwise introduce air into the liquid. The present invention is directed to the latter type of foamer, and the remaining discussion will accordingly focus on liquid stream foamers.
In an attempt to increase the level of turbulence within the mixing chambers of a foam producing device, the dimensions of the chamber and the dimensions of the orifice which permit the entrainment of air must be chosen carefully to provide the optimum combination of velocities, turbulence and atomization. U.S. Pat. No. 2,774,583 discloses an early attempt to carefully select orifice and chamber sizes, yet the presence of perforated screens was still necessary in order to obtain a foam having the desired small particle size.
Another problem faced by foam producing apparatus designers is the need to entrain a sufficient amount of air to mix with the quantity of liquid which must be delivered for the particular application, such as firefighting. Since the air is generally entrained by means of low pressure produced by the velocity of the liquid, the liquid must move at a relatively high velocity to produce the low pressures needed. For example, in U.S. Pat. No. 3,122,327, the foam forming liquid is introduced into the mixing chamber under high pressure, and is forced to pass through narrow orifices, thereby producing the high velocity necessary to draw sufficient atmospheric air into the mixing area by means of aspiration holes (column 3, lines 58-74).
One problem shared by the devices so far discussed is that the orifices which provide access to the mixing chamber are of a fixed size and tend to emit relatively constant amounts of air over wide variations of liquid flow. U.S. Pat. No. 3,188,009 discloses a series of clapper valves which open and close the aspiration orifices in response to the suction created by the high velocity liquid flowing through the mixing chamber. The amount of air introduced into the foam is therefore automatically adjusted according to the instantaneous fluid flow. A related problem which occurs in foam producing nozzles is known as "flooding". Flooding occurs when the liquid pressures become so high that the liquid is expelled outwardly through the air inlet orifices. U.S. Pat. No. 3,388,868 discloses a solution to this problem in which the air enters through inlet openings 26 and is then transported some distance through a duct 16 before coming into contact with the fluid in a mixing chamber, the fluid being conducted through a separate series of tubes 22. This arrangement apparently inhibits the foam forming action and a number of perforated screens and shields are required to produce foams having the desired characteristics.
The problem with the devices just discussed is that the air is introduced into the mixing chamber at a relatively low velocity, the velocity imparted to the air being caused only by the low pressure within the mixing chamber. An attempt to increase the entrained air velocity is disclosed in U.S. Pat. No. 3,799,450, where the inlet port is tapered so as to provide air of higher velocity and increase the area of contact between the air and liquid. The outer surface of the inlet port is a relatively large area and tapers to a very small orifice at the point where it enters the mixing chamber, U.S. Pat. No. 3,836,076 discloses a nozzle with an inclined annular surface formed on the inner periphery of the nozzle body. This surface is designed to deflect the stream inwardly to mix the foam producing agent with the gas which is present within the nozzle. The second embodiment of this patent uses a circular impingement disk to disrupt the flow and thereby generate foam.
Most relevant to the present invention is U.S. Pat. No. 3,822,217, in which water, air and detergent are introduced within a small cylindrical chamber to produce foam. The detergent is introduced to a tapering conduit and is entrained by the flow of water. The water/detergent mixture continues down the tapering conduit which abruptly changes its angle of taper into a larger expansion chamber. In the final stage, air is introduced to the water/detergent mixture inside a chamber having a uniform circular cross section. An alternate embodiment shows the air being introduced into the second stage of a tapering conduit having a compound angle.
U.S. Pat. No. 3,853,784 discloses a similar foam producing device in which an obstruction is placed at the point where the angle of taper of the mixing chamber abruptly increases. This obstruction is used to adjust the velocity of liquids passing through the chamber according to their different viscosities.
Another problem faced by the designer of a foam generating apparatus is, that in order to produce a uniform foam, there often must be some sacrifice in the velocity of the emerging mixture. Thus, although a uniform foam may be produced, the low velocity makes distributing the foam very difficult. A foam spraying device which attempts to address this problem is disclosed in U.S. Pat. No. 3,918,647. The foam sprayer disclosed therein provides a progressive control over the degree and quality of foaming action that can be achieved with an air aspirating type foamer by varying the angle of divergence of a liquid stream exiting from an orifice and directed to a pressure reducing passageway, including a sharply outwardly tapered portion terminated in a restricted throat passageway portion opening into an expansion chamber. The narrowest useful stream flowing from the orifice is a relatively concentrated liquid stream which initially strikes the walls of the throat passageway portion to produce a stream with a long throw accompanied by a modest degree of foam. By progressively increasing the angle of the stream flowing from the orifice, the stream becomes less concentrated and progressively more mist like, and strikes larger portions of the pressure reducing passageway, including the tapered portion thereof. An increase in foaming action occurs coupled with a reduction in the spray throw distance when the widest portion of the diverging stream exiting from the orifice strikes the end section of the tapered portion of the pressure reducing passageway. Such a spray pattern has been found generally to produce foam with good throw. However, even thicker foams can be achieved when the widest portion of the diverging stream initially strikes the pressure reducing passageway at points substantially behind the end section of the tapered passageway portion, but the progressively reduced throws which reach impractical magnitudes after only a small adjustment.
An improvement on the previous patent is disclosed in U.S. Pat. No. 4,013,228 wherein a longitudinal passageway in which pressure is reduced is physically moved in relation to the discharge orifice such that the characteristics of the diverging exit stream do not chamber with increasing foam viscosities.
Finally, U.S. Pat. No. 4,330,086 discloses a foam generating nozzle in which the expansion chamber is interrupted by a small pin which causes the foam to be deflected against the walls of the expansion chamber thereby promoting more thorough mixing.
The references just discussed, while sometimes satisfactory for their intended purpose, have left something to be desired in that they are either overly complex in design or else do not achieve the efficient mixture of components required to synthesize a satisfactory foam. The problem of mixing multiple components into a liquid stream which must then be mixed with air to produce a satisfactory foam, in a relatively passive device, has not been adequately addressed by these previous devices.